632 PHYSIOGRAPHY 



of submarine regions furnishes at least a partial answer. The sound- 

 ing-line in the Caribbean region has furnished examples of topo- 

 graphy due, as it seems, mainly to differential movements of blocks 

 of the earth's crust bounded by faults, which have not been modified 

 by subaerial denudation. In a similar way, as is to be expected, 

 a survey of other portions of the at present water-covered surface 

 of the lithosphere will supplement our knowledge of thee merged 

 portions of the same rock-envelope, and assist in an important 

 way in the deciphering of their histories. 



In the Arctic and Antarctic regions, where all is unknown, system- 

 atic research may be expected not only to extend many branches 

 of physiographic study, but to bring to the front as yet unsuspected 

 problems. 



The larger of the unexplored regions of the earth, however, are not 

 the only portions of our field of study that demand attention. New 

 ideas, new principles, and fresh hypotheses make an unknown country 

 of the most familiar landscape. The definite formulation of the base- 

 level idea, the suggestive and far-reaching principle embraced in the 

 term "geographic cycle," the planetesimal hypothesis as to the origin 

 of the earth, etc., furnish new and commanding points of view, or, as 

 they may be termed, primary stations in the physiographic survey of 

 the earth's surface, by means of which previous local surveys may be 

 correlated and corrected. 



In the search for problems, the unraveling of which may be ex- 

 pected to advance the scientific study of the earth's surface, the writ- 

 ings of travelers, the pregnant suggestions of those who make recon- 

 noissances into the realm of unknown facts and of unrecognized ideas, 

 as well as the precise and accurate pictures of portions of the earth's 

 surface presented on the maps of the topographer and the charts of 

 the oceanographer, point the way to still greater advancements, and 

 furnish inspiration to those who follow. 



Fundamental Concepts 



While physiography deals with the surface features of the earth, 

 the fact that in those features is expressed to a great extent the effects 

 of movements originating deep within the earth leads the student of 

 continents and oceans to ask of the geologist and the physicist 

 puzzling questions as to the changes that are taking place in the 

 great central mass of our planet, and even in reference to the origin 

 of the earth itself. So intimately are the various threads of nature- 

 study interwoven that the full significance of many of the surface 

 features of the earth cannot be grasped and their genesis explained 

 until the nature and mode of action of the forces within the earth 

 which produce surface changes are understood. 



